Electronic tone generator system



June 21, 1960 E. .1. HENLEY ELECTRONIC TONE GENERATOR'SYSTE'M 4 Sheets-Sheet 1 Filed Jan. 23, 1956 IN V EN TOR.

fDM/AKD J fifA/zfy c M ATTORNEV June 21, 1960 E. J. HENLEY ELECTRONIC TONE GENERATOR SYSTEM 4 Sheets-Sheet 2 Filed Jan. 23. 1956 -l in? INVENTOR v fawn/e0 J HENLEY ATTORNEV E. J. HENLEY 2,941,435

ELECTRONIC TONE GENERATOR SYSTEM 4 Sheets-Sheet :5

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June 21, 1960 Filed Jan. 23, 1956 INVEN TOR. 59/4/4120 I flax/m.

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- ATTORNEV June 21, 1960 E. J. HENLEY 2,941,435

ELECTRONIC TONE GENERATOR SYSTEM Filed Jan. 23, 1956 4 Sheets-Sheet 4 ATTORNEY My invention relates to electronic musical instruments, particularly to electronic organs and the like, and has for its .primary object to provide improved electronic tone production in electronic organs or in other musical instruments where it is desired to augment conventional production electronically.

Contemporary electronic musical instruments synthesize different tone qualities by combinging a few basic tone qualities in various proportions. The wave shapes of these basic tone qualities may be sine, sawtooth, or various complex shapes derived by mechano-electrical transduction or by non-linear distortion of sine waves. Complex waves are also modified by passing through formant filters.

It is, therefore, a further object of my invention to produce electrical waves having greaterutility as basic units in the synthesis of desired tone qualities.

It is another object of my invention to ecnomically synthesize new tones which are not presently available nor economically practical in contemporary electronic musical instruments.

It is still another object of my invention to maintain control of phase relationships in the output wave such that amplifier power output requirements may be greatly reduced.

It is yet another object of my invention to produce improved tones corresponding to those produced by organ pipes covering the range from about 16.3 to about 30.9 cycles per second, thereby making practical an electronic unit for extending the low-frequency range of one or more pipe organ stops.

'It is still a further object of my invention to produce a wide range of tone qualities not requiring the use of formant filters which tend to impart a common quality and upset the tonal balance of many electronic musical instruments.

Other and further objects will be obvious upon an understanding of the illustrative embodiment about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

In accordance with the present invention the foregoing objects are generally accomplished by providing a system for producing electrical waves wherein the fundamental wave is substantially eliminated and wherein only harmonics of the fundamental pitch corresponding to the frequency of the desired musical tone are present. It is thus possible to treat these waves and the fundamental oscillator output waves independently with regard to amplitude and phase and to combine them with each oth er and with other waves having different harmonic struc-' tures.

A preferred embodiment of the invention has been chosen for purposes of illustration and description and is shown in the accompanying drawing, forming a part of the specification, wherein:

United States Patent output wave.

instrument according to my invention.

Fig. 2 is a schematic diagram of an oscillator shown in Flg. 1 for one note, five arrangements for harmonic generation, an out-phaser for combining the sine wave oscillator output with the harmonic-containing wave inphase-opposition, and preamplifiers for amplifying the fut-phaser output and the sine wave output of the oscilator.

Fig. 3 illustrates one means of reinserting the sine wave fundamental in the desired amount before the signal passes to a stop control switch.

Fig. 4 illustrates an arrangement similar to Fig. 3 except that a simple filter is included for modifying the strength of the sine wave according to frequency.

Fig. 5 illustrates means for combining more than one complex wave with the desired amount of fundamental before the signal passes to a stop control switch.

Fig. 6 illustrates means for applying an electronic vibrato to an oscillator.

Fig. 7 illustrates typical wave forms present at various points in the circuit when one of the oscillators illustrated by Fig. 1 is put into operation.

Referring now to the drawings, particularly to Fig. 1, there are shown three oscillators 10, 11, and 12,- each of which may be a conventional Hartley oscillator. Each oscillator generates a signal of substantially sine wave shape and constant frequency within the requirements of this application. A complete organ bank would call for sixty-one or seventy-three or eighty-five of such oscillators for a manual division, and thirty-two or fortyfour oscillators for a pedal division, that is, one oscillator being required for the pitch corresponding to each note to be produced. A low-frequency accessory for a pipe organ may call for only twelve such oscillators.

As seen in Fig. 1, the output of each oscillator traverses several paths. One of these paths leads to a resistive network 46 which regulates the strengths of the sine waves fed to busses B6 and B16. The other paths lead to nonlinear semi-conductor harmonic generating devices 41, 42, 43, 44, and 45. Each of these devices has two output paths leading to separate busses. The plurality of paths permits a wide range of scaling to be accomplished by adjusting the outputs so that, for example, oscillators operating at progressively higher frequencies cause pro-- pressively weaker signals to be delivered to busses B1, B2, B3, B4, B5, and B6. Busses B11, B12, B13, B14, B15, and B16 carry stronger signals with increasing oscillator frequency. In this embodiment, the scaling ha been set at slopes of 3 db per octave.

The harmonic generating devices 41, 42, 43, 44, and

45 are well known to those skilled in the art but maybe reviewed briefly because of the subsequent circuit action and the special considerations in the selection of components. Referring to Fig. 2, device '41 is hown as a self biasing diode circuit which delivers signals to busses B1 and B11 through resistors R4 and R5. Con.- duction through the diode builds up a charge on capacitor C9 such that conduction during succeeding cycles occurs only during the short periods of time that the oscillator voltage exceeds the charge on C9. Resistor R6 regulates the discharge time of C9. Device 42 is shown as a silicon carbide varistor CR2 delivering its output to busses B2 and B12 respectively through resistors R7 and R8. Normally the generating device. -42 is used to generate odd order harmonics, but in this case the operating point has been shifted to modify the symmetry of the output wave by applying a biasing potential to busses B2and B12 respectively through resistors R30 and R31. This causes even, as well as odd, harmonics to appear inthe Device 43 is similar to device but without biasing potential. Therefore, the outputs delivered to busses B3 and B13 respectively through resistors R9 and R10 contain odd order harmonics and are substantially devoid of even order harmonics. Device 4.4 is shown as a crystal diode rectifier CR4 delivering outputs to buss'es B4 and B14 through resistors R11 and R12, respectively. A biasing potential is applied to these busses respectively through resistors R32 and R33, so that conduction through the diode occurs during less than one-half cycle of the oscillator output signal. Device 45 is a crystal diode rectifier CR5 delivering outputs to busses B5 and B15 through resistors R13 and R14, respectively.

It is. desirable that all crystal diodes be as nearly perfect rectifiers as possible. In this respect, most types of point contact or welded germanium or silicon diodes are superior to other diode types, such as selenium or copper oxide surface contact rectifiers. Silicon carbide varistors are difficult to control in manufacture and must be graded for uniformity before use in these circuits. it is also required that they be miniature types having low shunt capacities.

Referring again to Fig. 1, it will be seen that the busses B1, B2, B3, B4, and B5 connect to separate outphasers 51, 52, 53, 54, and 55, respectively, and are therein combined with the signals froma bus B6 carrying sine waves with the same scaling. The outputs of the outvphasers are amplified by one stage preamplifiers 61, 62, 63, 64, and 65, respectively. The sine bus B6 is also connected to a preamplifier 30, the output of which is further amplified by a preamplifier 60. The outputs of the preamplifiers 60, 61, 62, 63, 64, and 65 are combined in various desired combinations, as shown, in stopregulating networks 70, 71, 72, 73, 74, and 75. The network outputs are connected to the input of amplifier 90 via'stop switches 83, 84, 85, 86, 87, and 88, as required. The amplifier 90 may be any conventional high gain type and its output is connected to a loud speaker 91 which may be any type suitable for the frequency and power range to be covered.

The component part values used in the stop-regulating networks illustrated by Figs. 3, 4, and 5 are selected on the basis of individual taste in voicing and may be made fixed orvariable, as desired. The output circuit of each stop-regulating network contains a potentiometer to regulate the strength of the stop. Isolating resistances are placed in each output lead so that undesirable loading eifects will be minimized when several of the stop switches 83, 84, 85, 86, 87, and 88, are operated at the same time to connect the signals to a common amplifier input.

Vibrato may be applied to the oscillators by means of elements 20, 21, 22', and 23, as shown in Fig. 1. Referring now to Fig. 6, the essential portions of this vibrato circuit are shown in schematic form. A five to seven cycle per second signal from a vibrato oscillator (not shown) is applied to potentiometer R61. The movable arm P1 of this potentiometer connects to, a grid G of a vibrato control tube V6. Switch 89 connects the grid G to ground to disable the vibrato when it is not required. The voltage drop, because of plate current, through resistor R62 is such that approximately 100 volts DC. is applied to a vibrato bus B7. When switch 89 is opened, the vibrato oscillator signal applied to the grid G causes the plate current to fluctuate, thus causing the DC. potential on bus B7 to fluctuate at the vibrato rate. Vibrato capacitor C20 of circuit 20 has a barium titanate dielectric and is voltage sensitive. The fluctuating voltage on bus B7 accordingly causes the capacitance of the vibrato capacitor C20 to fluctuate, thereby frequency modulating the oscillator to which it connected. Capacitor C23 is chosen to have high rcactance at the vibrato oscillator frequency and a low 4 reactance at the tone oscillator frequency, thereby providing a suitable bypass for bus B7.

The unique features and principles of operation of my invention are best explained with respect to the problem of synthesizing sustained tones, such as pipe organ tones, although it is apparent to those skilled in the art that my invention is not limited to such tone production. With the use of filters to control the application of operating potentials, it, is possible to simulate many wind instruments, stringed instruments, or percussion instruments.

Many researchers have analyzed the acoustic spectra of organ pipes and found that in many cases the harmonics were comparable to or stronger than the funda mental component. This is also true of many other musical instruments. Therefore, it follows that the wave forms of these tones are necessarily complex and not easily derived by simple distortion of sine'waves, nor are they obtainable from filtering harmonic-rich Waves, such as pulse or sawtooth wave forms. A high pass filter applied to such waves to attenuate the fundamental component would overly emphasizethe upper harmonics and would not lend itself to the production of a harmonic flute tone, for example, a tone which contains low order harmonics in great strength.

Fig. 7 illustrates wave forms which may be observed at various points in the circuit by means of an oscilloscope. These are described with reference to Fig. 1. Wave forms W1, W2, W3, W4, W5, and W6 represent signals passing through three stages in the synthesis of organ tones. Reading from top to bottom, column A shows wave forms W1, W2, W3, W4, W5, and W6 presout on busses B1, B2, B3, B4, B5, and B6, respectively. Column B shows the corresponding wave forms present at the outputs of out-phasers 51, 52, 53, 54, and 55 and at the output of preamplifier 30, respectively. These wave forms of column B are of particular interest because they are the basic units from which various tones are synthesized. With the exception of the last wave form, that is, of preamplifier 3 they represent signals containing harmonics in great strength but practically no energy at the fundamental frequency of the oscillator. It is, therefore, to be noted that the fundamental frequency has been substantially eliminated and can be later inserted in any desired amplitude or phase. In this connection use of a vacuum tube to obtain a phase reversal has long been known and one method of use as an out-phaser in musical tone production has been described by W. E. Keck in United States Patents Nos. 2,148,478 and 2,233,948. It is of particular interest that I have not only used it in a different method of tone production but there are distinct advantages to. my method of connection. Amplification of the desired harmonic products is effectively obtained coincident with cancellation of the fundamental. Tube failure will result in total loss of signal rather than a disagreeable change in tone quality and overloading of the amplifier and speaker that would occur if only one pathfwere connected to the out-phaser.

Referring again to column B, wave form W1 is a pulse wave containing an extended series of odd and even harmonics; W2 is a wave containing the lower order of odd and even harmonics, principally the'second and third, with higher harmonics present in diminishing strength; W3 contains principally the third harmonic with higher order odd harmonics present in diminishing strength; W4 is similar to W1 except that the harmonic series does not extend as far. in wave form W5 only even harmonicsv are present in sigificant amounts. This has now become a new tone having an octave relation ship to the fundamental pitch of the oscillator. The second harmonic is now the fundamental, the fourth harmonic has become the second harmonic,-and the sixth harmonic has become the third, etc. The tone thus created bears an excellent resemblance to'th'e' pipe organ;

VioI'in DiapaSon played at four foot pitch and is an extremely valuable asset to the tonal resources of any organ. When using this wave form, the fundamental sine wave connection is not established for reinsertion of the fundamental at the stop control network 75. Wave form W6, column B, illustrates the reversed relationship of the sine wave after it has passed through the first preamplifier. However, the harmonic energy of the other waves in column B is in phase with corresponding energy in the waves of column A. This is because of the outer-phaser circuitry wherein the harmonic-containing signal is applied to the cathode, as is seen in Fig. 2 for out-phaser 55. The signal appearing at the plate is in phase with the signal at the cathode. However, the sine signal applied to the grid is reversed in phase before appearing at the plate. When the relative amplitudes of the two signals are correct, there is substantial cancellation of the fundamental component, as shown by waves W1 to W inclusive of column B. It is evident that cancellation would also occur if the grid and cathode connections were reversed, but the arrangement shown is to be preferred as it eliminates cross coupling from the harmonic-containing to the sine circuits.

In column C are shown typical wave forms W1, W2, W3, W4, and W5 at the outputs of stop-regulating networks 71, 72, 73, 74, and 75, respectively. It should be noted that the phases of all signals have been shifted 180 degrees from those indicated in column B by virtue of having passed through preamplifiers 61, 62, 63, 64, and 65, respectively. Now it will be observed in column C that the sine wave component illustrated by W6 has the original phase, but the harmonics are reversed in phase with respect to column A. What were formerly bumps or spikcs" on waves W1 to W4 inclusive have now become indentations. This means that a smaller peak amplitude is required for a given power output and amplifier requirements may be thereby greatly reduced.

New tones may be synthesized by combining two or more signals in various proportions and, in cases where peak amplitudes due to phase relationships are not a controlling factor, the harmonic structure of output signals may be further modified by including formant filters in any input leg or the output leg of stop regulating networks 71, 72 73, 74, and 75.

It will be evident to those skilled in the art that a device according to my invention lends itself to production of an extremely wide variety of tones in which the harmonic series are predominantly odd or even over narrow or extended ranges, and in which the ratio of harmonics to fundamental may range from approximately zero to several hundred percent. In the special case of a low-frequency unit covering the range from about 16.3fm about 30.9 cycles per second, wave form W2 (column B) is particularly effective. This obtains from the well known fact that the ear will supply the fundamental component if a natural series of harmonics is heard in favorable strength and proportions. Since the lowest-frequency component of significance in wave form W2 (column B) would be about 32.7 cycles per second, it is possible to fully utilize the power capabilities of amplifiers and loudspeakers designed to respond down to this frequency. The tone produced by a wave having the shape of wave form W2 (column B) is not unlike that of a Diapason tone produced by an organ pipe up to 32 feet long. When using this wave form, the fundamental sine wave connection is not established for reinsertion of the fundamental at the stop control network 72.

I have shown how useful basic electrical wave forms may be generated and how two or three of these wave forms may be combined in various proportions to synthesize. a wide variety of new tones. It would be presumptuous to say that a tone quality having a given name Should have a specific structure, as there are 'as many Resistors R1 ohms 220,000 R4 do 22,000 R5 do 33,000 R6 do 220,000 R7 do 68,000 R8 do 100,000 R9 do 68,000 R10 do 100,000 R11 do 100,000 R12 do 150,000 R13 do 100,000 R14 n 150,000 R15 d 220,000 R16 do 330,000 R17 d 5,000 R19 do 2,000 R20 do 47,000 R21 do 220,000 R22 do 2,000 R23 do 47,000 R24 do 5,000 R25 do 2,000 R26 do 47,000 R27 do 220,000 R28 do 2,000 R29 do 47,000 R30 do 100,000 R31 do 100,000 R32 do 100,000 R33 do 100,000 R34 megohms variable 1.0 R35 do 1.0 R36 do 1.0 R37 megohms 1.0 R38 megohms variable 1.0 R39 o 1.0 R40 do 1.0 R41 -.1 do 1.0 R42 megohms 1.0 R43 megohms variable 1.0 R44 do 1.0 R45 do 1.0 R46. --dO-. 1-0.

organ, tone generators, such as I have described, are' operated from a console having one or more manual keyboards, each with up to sixty-one keys, and a pedal keyboard with up to thirty-two keys. Each of these keys (not shown) operates an electrical contact corresponding to switches 80, 81, M82 shown in Fig. 1. In addition,

according to well known practices, each key may operate similar contacts which supply power to oscillators having octave or other relationships to the unison or normal pitch, and still other contacts are used to operate'from.

one keyboard the oscillators normally associated with another keyboard. It is evident from Figs. 1 and 2 that all basic wave forms are at once created coincident with the operation of the oscillators. However, only those tone qualities which are desired are made audible by operation of the desired stop control switches 83, etc., which connect the outputs of the stop control networks 70,, etc., to the amplifier and speaker system 91. In the specialized case of an electronic accessory for, a pipe organ, switches 80, 81 and 82 are normally 12 volt keying relays operated from contacts in the organ console.

An example of component part values for a representative embodiment is as follows (for 440 cycles per second operation):

R41 -e -a:"en-n.'-.:rnegolims- 1.0 R61 do 1.0 R62 vohms r 10,000 R63 V do 250 R101 ,do 270 R102 do 270 R103 do 270 R104 "do"-.. 270 R105 "do"-.. 270 R106 do- 10,000 R111 do 270 R112. dO 270 R113 do 270 R114 do 270 R115 do 270 R116 ..do 10,000

Capacitors Microfarad C1 1.08 C2 0.1 C3 2.0 C4 100 C5 0.25 C6 0.25 C7 0.25 C8 0.25 C9 0.05 C11 0002 C12 0.002 C20 0.1 C23 1.0 C63 20.0

1 lldllcroflarad voltage-sensitive ceramic capacitor.

Vacuum tubes V1 1 /2 type 12 AU7 V2 /2 type 12 AU7 V3 V2 type 12 AU7 V4 /2 type 12 AU7 V5 V2 type 12 AU7 V6 type 12 AUG Semi-conductors CR1 germanium diode CR2 silicon carbide varistor CR3 silicon carbide varistor CR4 germanium diode CR5 germanium diode Others will find my invention adaptable to many conditions of use and to the production of tones resembling those produced by many types of instruments. As various changes may be made in the form, construction, and arrangement of the parts herein, without departing from the: spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all mattersa-re to be interpreted as illustrative and not in any limiting sense.

What is claimed is:

1. A musical instrument for producing a sound wave having a predetermined tonal characteristic comprising, in combination, an oscillator for generating a wave of substantially smooth shape and of a frequency corre sponding to a fundamental component of the sound wave, means for establishing at least two paths from. an output of said oscillator, means for generating harmonics in one of said paths, means. for combining said one path and another of said paths in phase opposition, whereby said fundamental component. is cancelled and said harmonics are retained, meansfor reinserting and combining the fundamental component of the output of said oscillator at a predetermined strength with said harmonics,

whereby a resultant wave form. having the desired characteristics is obtained, an amplifier for amplifying said resultant wave, and a loudspeaker responsive to, said,

plified, resultant wave form.

2. A musical instrument for producing a sound wave having a predetermined tonal characteristic comprising, in combination, an oscillator for generating a wave of substantially smooth shape and of a frequency corresponding to a fundamental component of the sound wave, means for establishing at least two paths from an output of said oscillator, means including a non-linear semiconductive device for generating harmonics in one of said paths, means for combining said one path and another of said paths in phase opposition, whereby said fundamental component is cancelled and said harmonics are retained, means for reinserting and combining the funda-. mental component of the output of said oscillator at a predetermined strength with said harmonics, whereby a resultant wave form having the desired characteristics is I obtained, an amplifier for amplifying said result-ant wave, and a loudspeaker responsive to said amplified, resultant wave form. a v 3. A musical instrument for producing a sound wave having a predetermined tonal characteristic comprising,

grid and cathode of said discharge tube for cancelling said fundamental component, means for reinserting the fundamental component of the output of said oscillator,

at a predetermined strength, whereby a resultant wave form having the desired characteristics is obtained, an

amplifier for amplifying said resultant wave, and a loudspeaker responsive to said amplified, resultant wave form. 4. A musical instrument for producing a sound wave having a predetermined tonal characteristic comprising,

in combination, an oscillator for generating a wave of substantially smooth shape and of a frequency corresponding to a fundamental component of the sound wave, means for establishing at least two paths from an output of said oscillator, a non-linear semi-conductive device for" generating harmonics in one of said paths, a thermionic discharge tube having at least an anode, a cathode, and a grid for combining said one path and another of said paths in phase opposition, said two preceding Paths being connected respectively to the grid and cathode of said discharge tube for eliminating said fundamental component and retaining said harmonics, means for reinsertingf the fundamental component of the output of said oscilla-' tor at a predetermined strength and combining said funda mental component with said harmonics, whereby a resultant wave form having the desired characteristics is obtained, an amplifier for amplifying said resultant wave, and a loudspeaker responsive to said resultant wave form. i

5. musical instrument in accordance with claim 4, wherein said one path containing said semi-conductive device is connected to said cathode and said other path is connected to said grid of the discharge tube.

6. A musical instrument in accordance with claim 4,. wherein said semi-conductive device includes a linear impedance in series with a non-linear semi-conductor.

7. A musical instrument in accordance with claim 6,,

wherein said semi-conductor is a diode.

8. A musical instrument in accordance with claim 6, wherein said semi-conductor is a non-polar varistor.

9. In an electronic musical instrument. which produces a sound wave of predetermined tonal characteristic'gan oscillator for generating a wave of. substantially smooth shape and of a frequency correspondin to a'mfu'nda'w mental component of the sound wave, means for obtaining an electrical wave having a fundamental component and harmonic components in which the fundamental component has a peak amplitude less than the peak amplitude of the harmonic components thereof, said means including two paths from. an output of said oscillator, 21 nonlinear semi-conductive harmonic generating device in one of said paths, a thermionic discharge tube having at least an anode, a cathode, and a grid for combining said two paths in phase opposition, one of said two paths being connected to the grid and the other of said two paths being connected to the cathode of said discharge tube for eliminating said fundamental component and retaining said harmonic components, means for reinserting the fundamental component of the oscillator output at a predetermined strength, whereby a resultant wave form having said predetermined tonal characteristics is obtained, an amplifier, a loudspeaker, and means for energizing the loudspeaker according to said resultant wave form.

10. In an electronic musical instrument according to claim 9, wherein said means includes three paths from the oscillator output with a harmonic generator in each of two of said paths and wherein each of said two preceding paths are combined in phase opposition by separate thermionic discharge tubes with the third path of said three paths, said third path containing the fundamental component of the sound wave.

11. A musical instrument for producing a sound wave having a predetermined tonal characteristic comprising, in combination, an oscillator for generating a wave of substantially smooth shape and of a frequency corresponding to a fundamental component of the sound wave, means for establishing a plurality of paths from an output of said oscillator, at least two non-linear semiconductive devices for generating harmonics in at least two of said paths, at least two thermionic discharge tubes each having at least an anode, a cathode, and a grid, each of said paths with a harmonic generating device and each of said paths having the fundamental component being connected respectively to the cathode and grid of each of said discharge tubes for eliminating said fundamental component and retaining said harmonics in said paths, means for reinserting the fundamental component of the output of said oscillator at a predetermined strength and combining said fundamental component with each path containing said harmonics, whereby at least two resultant wave forms having the desired characteristics are obtained, an amplifier for amplifying selected ones of said resultant waves, and a loudspeaker responsive to said resultant wave forms.

12. In an electronic musical instrument for producing a plurality of sound waves corresponding to the notes of a musical scale, a plurality of oscillators for generating waves of substantially smooth shape and of frequencies corresponding to fundamental components of associated sound waves, a group of paths from an output of each of said oscillators, said paths of one group being connected in multiple with like paths of other groups, a non-linear semi-conductive device for generating harmonics in all of said paths except one in each group, a plurality of thermionic discharge tubes each having at least an anode, a cathode, and a grid for each of said harmonic generating devices, a bus connected to an output of each of said harmonic generating devices, a bus connected to said one path having said fundamental component, each of said harmonic containing buses being connected separately to the cathode of one of said discharge tubes, said one bus with fundamental component of said oscillators being connected in multiple to the grids of all of said discharge tubes, whereby said fundamental components are eliminated at outputs of said discharge tubes and said harmonics are retained, means for amplifying the outputs of said discharge tubes, means for selectively reinserting a fundamental component at a predetermined strength in each of said amplified outputs, and a loudspeaker responsive to said amplified outputs.

13. In a electronic musical instrument according to claim 12, wherein a biasing potential is applied to at least one of said buses connected to the harmonic generating devices.

14. In an electronic musical instrument according to claim 12, a vibrato bus connected to said oscillators, and means including an oscillator for selectively adding vibrato to said mentioned oscillators.

15. In an electronic musical instrument according to claim 12, wherein control means are provided for energizing said oscillators.

16. In a electronic instrument for producing a plurality of sound waves corresponding to the notes of a musical scale, a plurality of oscillators for generating waves of substantially smooth shape and of frequencies corresponding to fundamental components of associated sound waves, a group of paths from an output of each of said oscillators, said paths of one group being connected in multiple with like paths of other groups, a non-linear semi-conductive device for generating harmonics in all of said paths except one in each group, a plurality of thermionic discharge tubes each having at least an anode, a cathode, and a grid for each of said harmonic generating devices, a bus connected to an output of each of said harmonic generating devices, a bus connected to said one path having said fundamental component, an isolating resistance in said one path, each of said harmonic containing buses being connected separately to the cathode of one of said discharge tubes, said one bus with fundamental component of said oscillator being connected in mul tiple to the grids of all of said discharge tubes, whereby said fundamental components are eliminated at outputs of siad discharge tubes and said harmonics are retained,-

means for amplifying the outputs of said discharge tubes, means for selectively reinserting a fundamental component at a predetermined strength in each of said amplified outputs, and a loudspeaker responsive to said amplified outputs.

References Cited in the file of this patent UNITED STATES PATENTS 2,036,691 Gourau Apr. 7, 1936 2,130,251 Richards Sept. 13, 1938 2,148,478 Kock Feb. 28, 1939 2,199,702 Hammond May 7, 1940 2,233,948 Kock Mar. 4, 1941 2,287,105 Kannenberg June 23, 1942 2,386,892 Hadfield Oct. 16, 1945 2,463,597 Cahill Mar. 8, 1949 2,478,973 Mahren Aug. 16, 1949 2,486,208 Rienstra Oct. 25, 1949 2,498,337 Kent Feb. 21, 1950 2,506,723 Larsen May 9, 1950 2,547,759 Kent Apr. 3, 1951 2,568,644 Larsen et al. Sept. 18, 1951 2,599,510 Campbell et al. June 3, 1952 2,605,358 Neher July 29, 1952 

